Thermodynamic stability, phase separation and Ag grading in (Ag,Cu)(In,Ga)Se-2 solar absorbers

Gallium alloying and grading in Cu(In,Ga)Se-2 (CIGS) are well-established strategies for improving performance of thin-film solar cells by tailoring band profiles within the absorber. Similarly, Ag incorporation is considered to be an effective complementary route towards further advancement of the field. Herein, we explore thermodynamics of the formation of (Ag,Cu)(In,Ga)Se-2 (ACIGS) alloy. Using first-principles methods, we reveal the existence of a miscibility gap in the Ga-rich alloys at temperatures close to those employed for the co-evaporation growth. We demonstrate that this property can result in phase separation and the formation of Ag gradients throughout the film thickness. We prove experimentally that the phase separation can indeed occur during low-temperature growth and/or post-deposition treatments. Furthermore, we uncover the anticorrelation between Ag and Ga contents, and demonstrate thermodynamically-driven formation of [Ag]/([Ag] + [Cu]) gradients in films with a steep [Ga]/([Ga] + [In]) profile. Finally, we discuss how these phenomena can influence solar cell devices. The presented results are expected to provide fundamental insight into the physics of growth and processing of ACIGS absorbers, which could be utilized to further boost the efficiency of thin-film solar cells